The use of electrical signals to stimulate or steady heart rhythm (pacing) or to restore heart rhythm when the muscle fibers of the heart undergo very rapid irregular contractions, which result in very little pumping of blood (defibrillation), is a well accepted, lifesaving medical technique. Implantable cardioverter/defibrillator devices have been under development since at least the 1960's. The term cardioverter is used to mean a device for the correction of ventricular tachycardia (abnormally rapid heart rate of about 100-240 beats per minute) by discharging electrical energy into the heart. The term defibrillation is used to refer to high voltage shocks which terminate fibrillation (a rapid, chaotic heart rhythm resulting in no effective pumping of blood.) Implanted defibrillation is normally accomplished by passing a current between at least a pair of internally placed electrodes. The electrode arrangement may include a catheter or endocardial electrode which is transvenously positioned within the heart of the patient so that one of the electrodes is within the right ventricle. The other electrode, in the form of a flexible, substantially planar patch, is positioned outside the heart, either subcutaneously or within the thoracic cavity next to the left ventricle. Alternatively, the housing of the defibrillator may be used as an electrode. In other systems an electrode is positioned transvenously within the superior vena cava (SVC). The SVC electrode may be used in place of or in addition to the patch electrode. Electrical current is supplied to the electrodes by a battery powered pulse generator implanted under the skin of the patient, either in the abdominal or pectoral region. Improving the conductance path between the patch electrode (or device housing or SVC electrode) and the right ventricular (RV) electrode results in reduced energy required per defibrillation pulse and this may increase the lifetime of the system or allows for the use of smaller batteries.
For purposes of defibrillation, it is desirable to maximize the contact of the defibrillation electrode with the heart wall, preferably the septum between the right and left ventricles. Such intimate contact with the heart tissue makes defibrillation more effective by lowering the defibrillation threshold (DFT).
One prior art technique for positioning one or more defibrillation electrodes near the septum has used a lead system which includes a plurality of flexible electrodes which, when released, laterally expand into positions which bear resiliently against the surrounding heart walls. See PCT Application No. WO 89/06148 of Edhag. A similar system is disclosed in U.S. Pat. No. 4,998,975 to Cohen et al. These systems however, are somewhat complex and may be difficult to remove after chronic use. Additionally, the systems do not allow significant control in the placement of the electrodes.
A number of techniques have been developed for fixation of the distal end of transvenous endocardial leads within the heart of a patient. One such endocardial electrode is described in U.S. Pat. No. 3,902,501 to Citron et al. which uses a plurality of pliant fixation tines which extend at an acute angle to the lead body from the distal tip of the lead. When the lead is extended into the right ventricle, the tines act as an anchor catching in the trabeculae of the heart wall. Over time, the growth of tissue around the tines will further act to secure the lead tip in place. Another common prior art technique for lead fixation uses a helical or "screw" tip fixation device which extends from the distal tip of the lead body. A stylet or other mechanical means extending through the lead body is used to rotate the screw tip to cause it to bore into the heart tissue. Another fixation technique for a pacemaker lead is disclosed in U.S. Pat. No. 4,858,623 to Bradshaw et al. A rigid hook for engaging tissue is pivotally fastened to the lead in the vicinity of the electrode. The tip of the hook is normally resiliently urged into a recess in the lead adjacent to the electrode. A mechanism is coupled to the lead to permit the normal bias on the hook tip to be overcome to cause the hook to extend outward from the electrode. Each of these techniques is used to affix the distal tip of the lead body to the tissue of a patient's heart. However, with each of these prior art techniques, the positioning of the lead body is not accurately controlled, if at all.
Many of the prior art fixation techniques have been developed for use with pacemaker leads. With such leads, positioning of the distal tip of the lead is all that is required since the lead body is simply an insulated connector. Endocardial defibrillation leads, however, include a defibrillation electrode which extends along the lead body. Typically, the electrode of such prior art leads is fixated chronically by fibrosis or not at all and its placement is not accurately controlled at the time of implant.
It is an object of the invention to provide a transvenous lead system for use with an implantable cardioverter/defibrillator which allows precise electrode placement in intimate contact with the heart wall.
It is another object of the invention to provide a lead system having a fixation lumen for actuating a lead fixation device.
Other objects of the present invention will be apparent from the following description of the invention, read in connection with the drawings.